Please run the Orange installer (with required libraries) again. You should be prompted to install PyQwt5.If not please inspect the 'C:\Python27\Lib\site-packages\PyQt4' folder and make sure a directory (or file) named Qwt5 is not present there and then try running the installation again.

Thank you, the problem is solved: when I was installing Orange for the first time, I always checked Python 3.3 when I was asked. But the "main" Python on my PC is 2.7, so I've reinstalled Orange always selecting 2.7 and now everything is OK.